World demand for energy is expected to more than double by the year 2050 and to more than triple by the end of the century. Incremental improvements in existing energy technologies will not be adequate to supply this demand in a sustainable way. Sunlight represents by far the largest of all carbon-neutral potential energy sources. More energy from sunlight strikes the earth in one hour than all of the energy consumed on the planet in one year. However, currently, less than 0.1% of the world's electricity is generated using solar electricity generation. See “Basic Research Needs for Solar Energy Utilization, Report of the Basic Energy Sciences Workshop on Solar Energy Utilization” Apr. 18-21, 2005, U.S. Dept. of Energy, Office of Science.
The reason for the lack of widespread adoption of photovoltaic technology is directly linked with the high cost/watt of this technology. Industrial electricity today costs, on average, about 0.09$/kilowatt hour (kWh) in the United States. Solar electricity costs vary between approximately 0.2-0.5 $/kWh depending upon the system and climate. For photovoltaic technology to be competitive without subsidies, there consequently must be at least a 2-6× reduction in the cost per kWh. Improving on cost/watt for solar technology has two components: 1) reducing the cost of fabricating the device and 2) increasing the power conversion efficiency of the resulting device.
A substantial fraction of today's photovoltaic production is silicon (Si)-based. High efficiencies (i.e., greater than 20% in laboratory-scale devices) in single junction cells are only achieved in thick crystalline silicon devices. Due to the indirect band gap, which necessitates the thick absorber layer, and the associated high-temperature vacuum-based processing, Si is not an ideal material for an absorber layer as a result of the high materials and processing costs.
An alternative approach is to look at thin-film direct band gap absorber layers (rather than Si), which are typically metal chalcogenides offering a very high absorptivity for solar photons. The two principal metal-chalcogenide-based thin-film technologies are CdTe and Cu(In,Ga)Se2 (CIGS). CdTe cells with 16.5% efficiency and CIGS cells with 20.3% efficiency have been made. More recently, Cu2ZnSn(S,Se)4 (CZTSSe) devices have also been demonstrated with efficiencies approaching 10%. Often these materials can be deposited using low-temperature solution-based approaches, which are expected to be lower-cost than typical vacuum-based approaches. Photovoltaic devices based on chalcogenide absorber layers therefore offer substantial promise of being able to reduce materials and processing costs for solar technology. Devices based on CIGS and CZTSSe are particularly desirable because of the avoidance of the heavy metal Cd in the relatively thick absorber layer. Besides the cost of fabrication, achieving high efficiency in the thin-film chalcogenide-based solar cells is crucial for achieving low $/watt or $/kWh.
Therefore, cost-effective techniques for producing high efficiency photovoltaic devices would be desirable.